Over the past three decades a new spectroscopic technique with unique possibilities has emerged. Based on coherent and time-resolved detection of the electric field of ultrashort radiation bursts in the far-infrared, this technique has become known as terahertz time-domain spectroscopy (THz-TDS). In this review article the authors describe the technique in its various implementations for static and time-resolved spectroscopy, and illustrate the performance of the technique with recent examples from solid-state physics and physical chemistry as well as aqueous chemistry. Examples from other fields of research, where THz spectroscopic techniques have proven to be useful research tools, and the potential for industrial applications of THz spectroscopic and imaging techniques are discussed.

Terahertz spectroscopy and imaging – Modern techniques and applications

Terahertz (THz) spectroscopy, and especially THz imaging, holds large potential in the field of nondestructive, contact-free testing. The ongoing advances in the development of THz systems, as well as the appearance of the first related commercial products, indicate that large-scale market introduction of THz systems is rapidly approaching. We review selected industrial applications for THz systems, comprising inline monitoring of compounding processes, plastic weld joint inspection, birefringence analysis of fiber-reinforced components, water distribution monitoring in polymers and plants, as well as quality inspection of food products employing both continuous wave and pulsed THz systems.

Terahertz imaging: applications and perspectives

Terahertz time domain spectroscopy (TDS) was assessed as a nondestructive evaluation technique for aircraft composites. Damage to glass fiber was studied including voids, delaminations, mechanical damage, and heat damage. Measurement of the material properties on samples with localized heat damage showed that burning did not change the refractive index or absorption coefficient noticeably; however, material blistering was detected. Voids were located by TDS transmissive imaging using amplitude and phase techniques. The depth of delaminations was measured via the timing of Fabry-Perot reflections after the main pulse. Evidence of bending stress damage and simulated hidden cracks was also detected with terahertz imaging.

https://zenodo.org/record/1259241/files/article.pdf

Nondestructive evaluation of aircraft composites using transmissive terahertz time domain spectroscopy.

We review different techniques for modulation of the electromagnetic properties of terahertz (THz) waves. We discuss various approaches for electronic, optical, thermal and nonlinear modulation in distinct material systems such as semiconductors, graphene, pho- tonic crystals and metamaterials. The modulators are classified and compared with respect to modulation speed, modulation depth and categorized by the physical quantity they control as e.g. amplitude, phase, spectrum, spatial and temporal properties of the THz wave. Based on the review paper, the reader should obtain guidelines for the proper choice of a specific modulation technique in view of the targeted application.

THz Wave Modulators: A Brief Review on Different Modulation Techniques

Polymeric materials, in particular PVC, can find various industrial utilizations thanks to the use of plasticizers added during their processing. The most famous applications include wires and cables, coatings, flooring, paintings, packaging… After some generalities concerning plasticization theories and the description of plasticized petro- and bio-based polymers, this review details the well-known different petro-based plasticizers and more particularly phthalates which represent the most important category of PVC plasticizers. Owing to migration problems, impact on the human health and the environment, alternative candidates have been developed by researchers. Renewable resources and their wastes offer a large platform for the design of bio-based plasticizers using polysaccharidic or lipidic structures. In an in-depth analysis, the bio-based plasticizer structures, their groups and substituents (ester groups, alkyl chains, aromatic rings…) are gathered and examined in order to be able to predict their plasticizing efficiency and design new molecular and macromolecular plasticizers from natural resources. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 11–33

Petro‐based and bio‐based plasticizers: Chemical structures to plasticizing properties

We present the first investigation of plastic weld joints using terahertz waves. Terahertz time-domain spectroscopy clearly reveals contaminations like metal or sand within the weld joint of two high-density polyethylene sheets. Furthermore, areas can be identified where the welding process has failed and the parts to be joined are separated by a small air gap. We show that a three layer structure of polyethylene-air-polyethylene has a characteristic, frequency-dependent transmission behaviour. This allows for a distinction between welded and non-welded material as well as for the calculation of the air layer thickness from the relative transmission spectrum. Consequently, terahertz time-domain spectroscopy provides a promising new non-destructive and even contactless technique, which is desired by the plastics industry for detecting a variety of deviations from the ideal welding process.

http://www.jeos.org/index.php/jeos_rp/article/download/07013/189

Terahertz imaging: a new non-destructive technique for the quality control of plastic weld joints

We demonstrate the suitability of terahertz time-domain spectroscopy as a non-destructive, contact-free tool to monitor the glass transition in polymers – a core feature of the amorphous phase. Below the glass transition temperature Tg, segmental motions along the polymer chain are frozen due to the lack of free volume between neighboring macromolecules. We show that this transition also reflects in the temperature dependence of the refractive index at terahertz frequencies. Two domains can be identified, which differ in their sensitivity to temperature changes. To verify the proposed approach, we determine the glass transition temperature Tg of semi-crystalline poly(oxymethylene) (POM) with terahertz time-domain spectroscopy and validate the results by destructive differential scanning calorimetry (DSC) measurements.

Terahertz time-domain spectroscopy as a tool to monitor the glass transition in polymers

Monitoring the influence of different weathering conditions on polyethylene pipes by IR-microscopy

Terahertz (THz) spectroscopy holds a high potential as non-destructive, contact-free testing tool for the analysis of macromolecules, the monitoring of plastic processing and the inspection of plastic components. Even molecular properties, such as the glass transition temperature of highly-crystalline polymers, can be derived from THz measurements. Furthermore, we have identified a plethora of emerging applications in the plastics industry. To appeal to these upcoming challenges, we developed the first THz time-domain spectroscopy (TDS) system for the inline monitoring of compounding processes, which was awarded the Otto von Guericke Prize 2009. It has been demonstrated that the filler content at the end of the extrusion line could be precisely determined by time-of-flight measurements. In addition, THz technology is capable of measuring the water content sensitively, THz birefringence measurements reveal the fiber orientation in reinforced plastics and the quality of plastic weld joints or adhesive bonds can be inspected by the interference evaluation of the THz data. Due to the outstanding dielectric contrast of materials at THz frequencies, even contaminations invisible to x-rays or ultrasonic measurements can be revealed by THz TDS imaging opening the door for a new generation of non-destructive, contact-free quality control systems.

Benjamin Baudrit

Papers

Terahertz imaging: a new non-destructive technique for the quality control of plastic weld joints

Terahertz time-domain spectroscopy as a tool to monitor the glass transition in polymers

Monitoring the influence of different weathering conditions on polyethylene pipes by IR-microscopy

Terahertz spectroscopy: A powerful tool for the characterization of plastic materials

Analysis of the low-pressure plasma pretreated polymer surface in terms of acid–base approach